Department of Science and Technology Institutionen för teknik och naturvetenskap Linköping University Linköpings Universitet
SE-601 74 Norrköping, Sweden 601 74 Norrköping
LITH-ITN-KTS-EX--07/023--SE
Time based sequencing at
Stockholm Arlanda airport
Chun-Yin Cheung
Emin Kovac
LITH-ITN-KTS-EX--07/023--SE
Time based sequencing at
Stockholm Arlanda airport
Examensarbete utfört i kommunikations- och transportsystem
vid Tekniska Högskolan vid
Linköpings unversitet
Chun-Yin Cheung
Emin Kovac
Handledare Peter Larsson
Examinator Tobias Andersson
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This master thesis is the completion of our education to Master of Science in Communication and Transport systems at the Department of Science and Technology, Linköping University. The work comprises 30 ECTS and is executed at SAS Sverige. We would like to thank our supervisor Anna Norin and our examiner Tobias Andersson at Linköping University for their support. We would also like to thank our supervisors at SAS Sverige Per Ahl and Peter Larsson for their help and patience with us. Furthermore, we would like to address our gratitude to Mats Lindholm at LFV for providing us with valuable information. Other persons that have been helpful to us are Christer Forsberg at Avtech and Ulf Schack at LFV.
We want to express our appreciations to our opponent Joanna Beltowska.
Finally, we would like to thank all the pleasant people on our study visits to Stockholm ATCC and Green Approach real time simulation 2007 in Malmö for answering questions and contributing with valuable information to us.
Norrköping 2007 Cheung, Chun Yin Kovac, Emin
Abstract
Present Air Traffic Control System is functioning well, but is not always optimal, neither from an operative nor an environmental perspective. Therefore a new concept within the system is under development. The proposed concept is designated as time based sequencing of aircraft. In time based sequencing connections, the aircraft are categorised depending on their navigation capability. On the one hand there are aircraft capable of navigating with a high precision and perform optimal descent profiles, denoted as Green Approach aircraft in the report. On the other hand there are all the other aircraft with less advanced navigation ability, denoted as non-precision aircraft in the report.
Time based sequencing is about sequencing the arrival flow of aircraft in time. With the aid of CIES (a tool with the capability of sequencing the flow in time) the Air Traffic Control (ATC) sequences the arriving traffic in terms of time. The Green approach aircraft are sequenced directly to the runways and are supposed to land with a precision of ±10 s, while the non-precision aircraft are sequenced to the so called entry points (basis for non-precision aircraft) so that a passage occurs above the entry points with a precision of ±30 s whereupon they perform an as far as possible optimal descent. Time based sequenced approaches are more environmental friendly than traditional procedures. With an optimal descent profile performed by a Green Approach aircraft, approximately 100 kg of jet fuel is saved.
The intention of this thesis is to investigate which time deviations at the entry points for the arriving non-precision aircraft that can be accepted providing that aircraft are only controlled with speed adjustments (ordered by the ATC to the pilots) in order to avoid a transgression of the horizontal separation rules from entry point to landing. A simulation study is conducted with the intention of finding out which time deviations that can be accepted.
On the basis of the results of the simulation study a solution is proposed. The position of the three shorter entry points should be moved further away from the runway to at least the same distance as the longest as it has the largest acceptable EAT deviation (enough for the approaching precision of ±30 s) on the highest rate (34) in order to achieve larger acceptable EAT deviations for the other entry points. However, even if every entry point is on the same distance to runway as the uttermost, speed alternations are not sufficient to sustain the horizontal separation between aircraft. In case of higher rates than 34, the ATC would have to perform traditional procedures of managing arriving aircraft, i.e. leading the aircraft into S-curves. Hence, the proposal is to mix traditional approach procedures (allowing radar vectoring in order to guarantee separation) with time base sequenced approaches. If present positions of the entry points are kept, the result would be the same as above, except that the ATC would have to perform traditional procedures when the rate exceeds 28.
Contents
1
Introduction... 1
1.1 Background ... 1 1.2 Purpose ... 1 1.3 Problem definition... 1 1.4 Delimitation... 2 1.5 Methodology ... 2 1.6 Definition list... 3 1.7 Outline... 42
The present system... 6
2.1 The Air Traffic Control System ... 6
2.2 ATC system components ... 7
2.2.1 Communication equipment and procedures ... 8
2.2.2 Navigational equipment ... 8
2.2.3 Surveillance equipment... 8
2.2.4 The air traffic management system ... 8
2.2.5 Operating staff of the ATC system ... 9
2.3 Arlanda airport ... 9
2.3.1 The airport’s runway system... 9
2.3.2 Physical description of Arlanda airport ... 9
2.4 Air Traffic Control Centre (ATCC) Stockholm... 10
2.4.1 EUROCAT 2000... 10
2.4.2 Maestro ... 11
2.5 The system’s users – the aircraft ... 11
2.5.1 Separation minima ... 11
2.5.2 Flight Management System... 12
2.5.3 Precision Area Navigation... 13
2.5.4 Standard Terminal Arrival Route ... 13
2.5.5 The landing procedure... 14
2.5.6 Aircraft Communication and Reporting System – ACARS ... 16
2.6 Capacity... 17
2.7 Parallel runway operations ... 17
2.8 Shortages of the present system ... 18
3
Research ... 20
3.1 The NUP II+ project... 20
3.2 The Green Approach trials ... 20
3.2.1 Four-dimensional Trajectories – 4DTR... 22
3.3 Definition of time based sequencing for approaching aircraft... 22
3.3.1 Description of the concept ... 23
3.3.2 Collaborate Information Exchange System ... 25
4
Simulation theory... 27
4.1 Why simulation is an appropriate method for the subject... 27
4.2 What is simulation?... 27
4.2.2 Different kinds of simulations ... 28
4.2.3 Randomness in simulation ... 28
4.2.4 Replications... 28
4.2.5 Different parts of a simulation study ... 29
4.3 The simulation tool Arena... 31
4.3.1 Pieces of a simulation model in Arena ... 31
4.4 Arena elements... 32 4.4.1 The modules ... 32 4.4.2 Create... 32 4.4.3 Dispose... 32 4.4.4 Assign... 33 4.4.5 Delay ... 33 4.4.6 Separate ... 33 4.4.7 Record ... 34 4.4.8 Decide ... 34
4.4.9 Stations and transfer ... 35
4.4.10 Submodel... 35
5
Simulation study of time base sequenced approaches... 36
5.1 Simulation objectives ... 36
5.2 Conceptual model... 36
5.3 Modelling description and elements ... 37
5.3.1 P-RNAV STAR... 37
5.3.2 Horizontal separations ... 39
5.3.3 Input data ... 39
5.3.4 Deviations ... 40
5.3.5 Conflicts and speed adjustments... 41
5.3.6 Output of the model... 41
5.4 Modelling assumptions ... 42
5.5 Data quality ... 42
5.6 Verification... 42
5.7 Validation... 42
6
The model behaviour ... 44
6.1 Approach procedure ... 44
6.2 Model algorithm... 45
6.3 Model design ... 46
6.3.1 The Main model ... 46
6.3.2 The submodel ... 48
6.3.3 The Subsubmodels (1.1 – 1.83) ... 50
7
Scenarios ... 53
7.1 Description of the scenarios ... 53
8
Results ... 54
8.1 Results of scenarios involving only medium aircraft... 54
8.2 Results of scenarios involving 3 heavy aircraft ... 55
9
Conclusions ... 57
11
List of references ... 60
12
Appendices... 65
APPENDIX A Uniform distribution... 65
APPENDIX B Speed profile graph and table... 66
APPENDIX C Excerpt from LFV flight data... 67
APPENDIX D Simulation output data ... 69
TRS Medium only... 69
XILAN Medium only ... 70
ELTOK Medium only ... 71
HMR Medium only... 71 TRS 3 Heavy... 73 XILAN 3 Heavy ... 74 ELTOK 3 Heavy... 74 HMR 3 Heavy... 75
List of Figures
Figure 1 Schematic overview of the airspace division ... 7Figure 2 Positions of the runways at Stockholm Arlanda airport... 10
Figure 3 An example of Flight Management System ... 12
Figure 4 An example of a STAR ... 14
Figure 5 Projected view of the IAP... 15
Figure 6 A common arrival procedure of today... 16
Figure 7 Example of an S-curve seen from above... 16
Figure 8 Schematic overview of a Green Approach... 21
Figure 9 4DTR displayed on CIES ... 22
Figure 10 The concept of time based sequencing... 24
Figure 11 Approximate distance in time from reception of EAT to the entry point ... 25
Figure 12 CIES graphical user interface ... 26
Figure 13 The Create module and its settings... 32
Figure 14 The Decide module and its settings... 33
Figure 15 The Assign module and its settings ... 33
Figure 16 The Delay module and its settings... 33
Figure 17 The Separate module and its settings ... 34
Figure 18 The Record module and its settings... 34
Figure 19 The Decide module and its settings... 34
Figure 20 The Station and Route modules... 35
Figure 21 The route for the animation of the physical path... 35
Figure 22 The settings for the Route module... 35
Figure 23 The Submodel block... 35
Figure 24 Sequence of work ... 36
Figure 25 Conceptual model... 37
Figure 26 P-RNAV STARs used in the study... 38
Figure 27 Theoretical STAR... 39
Figure 28 Model algorithm ... 45
Figure 30 The process to create arriving entities ... 47
Figure 31 Assignments and aircraft type check... 47
Figure 32 The last separation check, assignments and the disposal of entities. ... 47
Figure 33 STAR decide module... 48
Figure 34 Overview of the submodel... 49
Figure 35 Overview of a subsubmodel ... 50
Figure 36 Transgression check section of a subsubmodel... 50
Figure 37 Speed adjustments and the speed check modules... 51
Figure 38 Speed recovery modules... 51
Figure 39 EAT deviation as a function of the rate (Medium aircraft only)... 54
Figure 40 EAT deviation as a function of the rate (3 heavy aircraft)... 55
Figure 41 Probability density function ... 65
Figure 42 Speed profile graph... 66
List of Tables
Table 1 Distance based separation minima rules by ICAO ... 11Table 2 MTOW for different aircraft types ... 12
Table 3 Distance to runway from the waypoints and speed directives for non-precision aircraft ... 39
Table 4 Distance to runway from the waypoints and speed directives for non-precision medium and heavy aircraft... 40
Table 5 Allowed EAT deviation for different entry points and rates (medium aircraft only) shown in ± seconds... 54
Table 6 Allowed EAT deviation for different entry points and rates (3 heavy aircraft) shown in ± seconds ... 55
Table 7 Speeds used in the model... 66
Acronym list
4D Four-dimensional (lat, long, altitude, time)
4DTR 4D trajectory
A-CDA Advanced Continuous Descent Approach
ACARS Aircraft Communication and Reporting System
ADS-B Automatic Dependent Surveillance – Broadcast
AMAN Arrival Management (Manager)
ATA Actual Time of Arrival
ATC Air Traffic Control
ATM Air Traffic Management
CDA Continuous Descent Approach
CFMU Central Flow Management Unit
CIES Collaborate Information Exchange System
CNS Communication, Navigation and Surveillance
DMAN Departure Management (Manager)
EAT Expected Approach Time
ETA Estimated Time of Arrival
FCFS First come first served
FMS Flight Management System
GNSS Global Navigation Satellite System
GPS Global Positioning System
ICAO International Civil Aviation Organisation
IFR Instrument Flight Rules
ILS Instrument Landing System
Kts Knots
LFV Swedish Civil Aviation Administration
MLS Microwave Landing System
NM Nautical Mile
NUP NEAN Update Programme
RETA Revised Time of Arrival
RNAV Area Navigation
RTA Required Time of Arrival
RWY Runway
STAR Standard instrument Arrival Route
TMA Terminal Area
1 Introduction
1.1 Background
1.2 Purpose
1.3
The introduction of Green Approaches at Stockholm Arlanda Airport started within the framework of the NUP II+-project in January 2005. The trials were executed during low peak periods, where the risk of disturbance by other aircraft was low. To be able to operate in heavier traffic load, the possibility of controlling the flow in terms of time has to be investigated. Time based sequencing is an approach to prepare Green Approach for heavier traffic loads which involves advanced calculations to determine Estimated Time of Arrival (ETA). The aircraft can thereby send its remaining flight profile known as a Four Dimensional Trajectory-data (4DTR) and possibly receive a Required Time of Arrival (RTA) from Air Traffic Control (ATC) far in advance if needed from a sequencing perspective. A hypothesis is that aircraft without adequate data link equipment and even aircraft equipped only with radio also can be sequenced with a lower but yet acceptable time precision by being assigned a time constraint on the entry point instead of at the runway.
This thesis aims at studying and defining the concept of time based sequencing in Air Traffic Management. Furthermore, the purpose is to seek conditions enabling an application of time based sequencing for all kinds of aircraft. The study is performed parallel to the present part of NUP II+-project carried out by SAS and the LFV Group.
Questions at issue:
• How large deviations from the Expected Approach Time1 (EAT) can
approaching aircraft not performing Green Approaches (non-precision aircraft) have?
• Which time precision is required for a specific capacity (number of landings per hour, i.e. the rate)?
Problem definition
Presently the aircraft management at Arlanda is well organized with a high safety level. Improvements can however be made as with many other technical systems. Especially since existing ATC routines originate from the industry’s infancy in the 1950s. One proposed and still under development way of improving the system is by introducing the concept of time based sequencing. Currently, time based sequencing is being tested only during low peak traffic and with certain kinds of aircraft, e.g. SAS Boeing 737 New Generation. Other aircraft are managed with traditional ATC procedures, see section 2.1. During the busy periods all arriving aircraft are managed according to existing ATC procedures.
A time base sequenced system presupposes that the non-precision aircraft are controlled towards an EAT. In this thesis EAT is equivalent to the time when aircraft are expected to pass several defined entry points, and not the time when the aircraft leave their holding procedures (queuing) as in the original definition. Hypothetically, the aircraft can be controlled to the entry points with a precision of ±30 s.
The assignment of the thesis is to investigate which time deviation to the entry points for the arriving non-precision aircraft that can be accepted if time based sequencing is introduced on 1 Expected Approach Time - See Section 1.6 Definition list
a broad front in all traffic densities, i.e. introduced as a rule for all kinds of approaching aircraft that have different navigation equipment with different precision levels. Arlanda has four entry points that constitute the base for the aircraft without capability of performing Green Approaches.
The time deviation is defined as how much time an aircraft can deviate from their EAT, i.e. how much earlier or later an aircraft can arrive at the entry point when it performs its descent operation. Practically two methods of affecting aircraft are at disposal for ATC in order to sustain or regain the horizontal separation rules for aircraft, either by radar vectoring2 of
aircraft or by ordering the pilots to adjust the speeds of the aircraft.
The thesis has its focus on the latter method. An aircraft during descent can in general be adjusted with one knot per second and maximally ±10 % of the speed that the aircraft have at a certain moment, in order to avoid a transgression of the horizontal separation rule from entry point to landing. The problem can be summarized by the following question: Which time deviation from EAT can be accepted at the entry points providing that aircraft can be affected according to the above description from entry point to landing?
1.4 Delimitation
1.5
The thesis is limited to only the landing processes with non-precision aircraft as prerequisites.
Methodology
In order to gain knowledge to achieve the purpose of the thesis, several methods are used. 1. Data collection of air traffic management and previous research by searching in
libraries and scientific reports.
2. Interviews with experts to gain an extensive understanding of the objective.
3. Participations at work meetings and simulation within the NUP II+ project to gain inspirations and directions of an approach to the investigation.
4. Simulation study.
2 Radar vectoring - See Section 1.6 Definition list
1.6 Definition list
These terms are occurring in the report. They are defined in this section with the intention of making it easier for the reader to understand the report.
ADS-B – Automatic Dependent Surveillance-Broadcast - a function on an aircraft or a ground
vehicle that periodically broadcast its identity, state vector (horizontal/vertical position and velocity) and other information derived from GNSS. ADS-B is automatic since no external stimulus is required to bring forth a transmission [1]. It is dependent as it relies on on-board broadcast transmission systems in order to supply other users with surveillance information.
B-RNAV - Basic Area Navigation - an application for Area Navigation where the demand on
navigation accuracy is RNP 5 which implies that the aircraft can navigate along a specified flight path with an accuracy of ± 5 NM during 95 % of the flight time [2].
Discrete-event simulation – is dynamic, discrete change and stochastic [3].
DME/DME - Distance Measuring Equipment - a radio receiver/sender on ground in
combination with sender/receiver on an aircraft that makes it possible on instrument of the aircraft to monitor the distance to the sender [2].
EAT - Expected Approach Time - a point in time when ATC expects an arriving aircraft to
leave hold to complete its approach for landing [4].
ETA - Estimated Time of Arrival - means different things depending on which type of
approach it is [5]. For IFR-flight (flight where the flight position is determined solely with the instrument of the aircraft) ETA means that point in time at which the aircraft is expected to land over the fixed point, defined in relation to navigation aid, from where the purpose is to start an instrument approach. If no navigation aid is used in conjunction with the airport ETA means the point in time when the aircraft is expected to arrive above the airport.
FL - Flight Level - a surface of constant atmospheric pressure which is related to a specific
pressure of 1013.2 hPa [6]. Flight level solves the problem with natural pressure variations in different regions, since the aircraft altimeter is essentially a calibrated barometer. All operating aircraft on flight levels calibrate to this standard setting regardless of the sea level pressure and therefore minimise the risk of collision.
FMS – Flight Management System – Navigation equipment for aircraft that determines the
position of the aircraft by collecting data from the speed and height measuring equipment [2].
GNSS – Global Navigation Satellite System – a definition by ICAO. GPS and other coming
satellite based navigation systems are all parts of GNSS [2].
ILS - Instrument Landing System - ground radio equipment that is used to, in conjunction with
instrument on an aircraft, it determines the position of an aircraft during the final approach expressed in altitude and sideways deviation from a nominal runway in order to receive some information about the distance to the point of touchdown [2]. ILS is used as an aid when precision approaches are performed.
INS - Inertial Navigation System - a system for position determination through continual
measurement of changes of an aircraft’s velocity and direction [2].
Instrument runway - a summarized designation of runways with installed equipment, with
several categories of precision, prepared for instrument approach [2].
Instrument approach – is a procedure which allows the pilot of an aircraft to land with
reduced visibility and to rely only to the instruments of the aircraft to initiate landing [2].
MLS - Microwave Landing System - ground equipment system for guiding the aircraft to
landings [7]. MLS has higher capacity than ILS and was intended to replace it. The introduction of satellite navigation caused that MLS was not introduced on a broad front.
Radar vectoring - “Provision of navigational guidance to aircraft in the form of specific
headings, based on the use of radar.” [8]
RNAV - Area Navigation - a navigation method that makes it possible for aircraft to follow an
optional flight path either within a stationary navigation aid’s cover area or within boundaries for a self contained aid’s possibilities or by a combination of them [9]. A self contained aid is an airborne navigation system that is independent of other aids outside the aircraft.
RNP - Required Navigation Performance - is information about an aircrafts navigational
accuracy in a defined airspace [10].
Required Time of Arrival (RTA) – is the capability of an FMS to reach a specified waypoint at
a specified time without external aids [11]. In a future system the use of RTA could lead to reduction of the level of Air Navigation Service Provider (ANSP) interaction and service required in certain situations as the FMS monitors and actively strives for to meet the RTA.
TMA - Terminal Area – a control area established for one or several airports.
VMC – Visual Meteorological Conditions – Weather conditions expressed in values for flight
sight, distance to clouds and cloud basis that are equal to or larger than determined minima [2].
1.7 Outline
This section contains short descriptions of the upcoming chapters:
2. Review and description of the present ATC system, the physical characteristics of the airport of Arlanda, the Stockholm Air Traffic Control Centre and aircraft, where among other things the landing process is covered. Finally, some shortages of the present system are presented.
3. The ongoing development within the ATC domain. The Green Approach concept with
4DTR downlink. The definition of time based sequencing that essentially is of crucial
importance for the development. The chapter is completed with a review of the Collaborate Information Exchange System – CIES, an aid that plays an important role for the concept of time based sequencing.
4. Definition of simulation and models, important aspects of simulation such as randomness and replications. Moreover, different parts of a simulation study are treated. The chapter
is concluded with the simulation tool Arena. Description of the modules used in the model.
5. The analytical part of the thesis. The chapter begins with a description of the objectives of the simulation study. It continues with the conceptual model, modelling description and assumptions, verification and validation.
6. Design of the Arena model. The modules, variables and parameters used in the model. 7. Scenarios. How the experiments are performed.
8. Results. Interpretation of the output data. 9. Conclusions.
10. Further work.
2 The present system
This chapter brings up topics concerning the present system as a background for the thesis.
2.1
The Air Traffic Control System
The ATC system is founded over a specific area, the airspace, to provide safe, efficient and regular air traffic [14]. This means that the system’s users, the aircraft, should be served without any conflicts among each other and the neighbouring terrain. In addition, the aircraft should be served without considerable delays that the ATC could give rise to. Moreover, a desire is to always have enough system capacity to meet expected demand. The goal is to provide a service for the aircraft for flying through the airspace between origin and destination along fuel and time optimal flight paths known as trajectories.
To accomplish the above specified operational attributes, the ATC divides the airspace into smaller portions at two levels [14]. On the first level, the airspace is divided into airport zones, terminal areas, and low/high altitude en-route areas. The second level is where all the first level zones are divided into smaller parts, so-called ATC sectors. Each sector is allocated to one or more air traffic controllers whose responsibility is to monitor and control the air traffic in their respective sector. To provide efficient management of arriving and departing traffic, airport zones around the airports are founded. The airport zones are horizontally spread over a radius of 65-80 km around the airports. Vertically, the boundaries of the airport zones are defined by the ground level and flight level (FL) 100 (each flight level corresponds to 1 000 ft which is approximately 305 m). The aircraft fly through this area along advised trajectories which are defined by radio navigational devices or radar vectoring by the ATC. Attributes of the aircraft that are always taken into consideration are their weight, approaching/departing speed and climbing/descending rate.
Low altitude areas (LAA) are founded above and around the Terminal Area3 (TMA) [15]. Vertically, the areas extend from 3000 m to 6000 m above middle sea level. The aircraft’s climbing and descending flight paths are established in this airspace. Descending aircraft use this area to go from cruising altitude to the lower altitude where they can enter the TMA. Climbing aircraft use the LAA in order to reach cruising altitude and enter an airway in the high altitude area. High altitude area is situated at heights above 6 000 m. Commercial aircraft use the high altitude area for cruising at flight levels along specific airways. In order to provide vertical separation between aircraft, constant flight levels are applied. Figure 1 shows a schematic description of the airspace division to control air traffic.
3 Terminal Area – See Section 1.6 Definition list
R – Runway System AZ – Airport Zone H – Holding Area TMA – Terminal Area LAA – Low Altitude Area HAA – High Altitude Area A/C - Aircraft
Figure 1 Schematic overview of the airspace division
Aircraft flying on the same flight level are horizontally separated, by ATC, by time and/or space based separation rules [15]. The aircraft have different performances that may have an impact on the air control process. Example of such performances could be the most economical speed and altitude, climb/descent rate and available navigational equipment. During traffic congestions or bad weather, arriving aircraft may be asked to delay their approach [16]. The procedure is known as holding. Occasionally, when delays are expected, the aircraft can be held at a high level far away from the airport. Several holding patterns are present near main (large) airports. Aircraft fly along a “track” until an approach is possible. Holding patterns in the near vicinity of airports are sometimes called “stacks” due to that the aircraft are stacked one above the other with 1 000 ft separation. As the lowest flight is cleared to leave the hold, other aircraft are lowering their altitude in steps by 1 000 ft. This kind of sequencing is known as first come first served (FCFS). A delay less than 20 minutes is considered as short by the present air traffic control. In cases where the delay is longer, all the concerned pilots will be given an indication of how long the hold will last and will be given an EAT which is the time the flight can expect to leave the lowest level of the holding stack and start its approach to the airport.
2.2 ATC system components
An ATC system consists of communication, navigation and surveillance (CNS) equipment, air traffic management and staff operating it.
2.2.1 Communication equipment and procedures
Communication equipment is composed of communication channels used for transmission of information between the pilots and air traffic controllers (VHF/UHF air/ground voice and non-voice communication links), communication links between specific ATC’s control units and communication links exchanging information between the ATC and its environment [17]. The International Civil Aviation Agency (ICAO) has issued procedures for radio telephony phraseology in English which are followed by almost all countries in the world [18]. The procedures are described and illustrated in documents provided by ICAO. It is of great importance that pilots and controllers interpret the ATC language correctly. There are several cases where risk situations have arisen due to partly careless radio telephony procedures. All aircraft within the responsibility area of a specific controller receive transmission from the controller. Furthermore, the pilots of the aircraft can hear transmission of all other aircraft using the same frequency. Before establishing a contact with a controller, the pilot must listen to a specific frequency to assure that there are not any other transmissions occurring. When this is certain the pilot will try to contact the controller several times. In case of failing to establish a contact, the pilot will return to the last known working frequency and inform that contact could not be established. The controller will try to take contact with the next sector and give the pilot a new radio frequency.
2.2.2 Navigational equipment
Navigational equipment is composed of ground aids and airspace satellites [17]. The two aids are used for the aircraft primary navigation. Examples of aids are: external overall en-route aids, airport external navigational equipment (the approach lighting systems, airport slope indicators and the airport surface detection equipment) and external above water en-route aids.
2.2.3 Surveillance equipment
Surveillance equipment is composed of two radar systems; primary radar and beacon (secondary) radar [17]. With radar the ATC can monitor the traffic situation on the radar scope, maintain visual control of separation between the aircraft and make proper decisions which are sent to the pilots via communication link.
2.2.4 The air traffic management system
The Air Traffic Management (ATM) systems consist of subsystems such as airspace management, air traffic services and air traffic flow management [17]. Airspace management has its focus on the availability of airspace to accommodate system user’s requirements for optimum airspace. Air traffic services, which are a core component of the ATM system, are used by ATC to attain its basic objectives which are to continuously provide safe, efficient and regular movement of air traffic. Air traffic flow management aims to optimise the air traffic flows movement, lessen the cost of airborne as well as ground flight delays and prevent system overloads. In European air traffic there is a Central Flow Management Unit (CFMU), whose role is to ensure traffic safety and efficiency of all flights by facilitating the management of the network [19]. All international flights within near whole Europe (36 states) have to send their flight plans to this central unit before departing. CFMU has mandate to, among other things, settling demand and capacity by applying rerouting and delayed take-off times.
2.2.5 Operating staff of the ATC system
The operators of the ATC system are the air traffic controllers and the pilots [17]. As mentioned above, air traffic controllers are responsible for monitoring and controlling air traffic in their respective zones. There are three types of air traffic controllers; tower controllers, TRACON (Terminal Radar Approach Control) controllers and en-route controllers at ARTCCs (Air Route Traffic Control Centres) [20]. The airport controllers handle the traffic in the near vicinity of the airport. Their responsibility is to manage flights landing at or departing from the airfield and to control movement of aircraft while they are on ground. In regard to approach controllers, who handle flights out from the airfield before they are handed over to the airport control, are responsible for the safety of aircraft approaching and departing from airport between 16 km and 80 km from major airports. The area controllers’ responsibility is to deal with air traffic at higher altitudes (en-route traffic). Their airspace is split into sectors which are three dimensional blocks of airspace of defined dimensions. In general, air traffic controllers may be organized into teams consisting of one to three members [17]. Cockpit crews of medium or large aircraft consist of two or three members depending on the aircraft type. Newer aircraft types usually consist of two members. The pilots use the communication links and avionics to navigate the aircraft.
2.3 Arlanda airport
Arlanda airport is an international airport located 40 km north of Stockholm. It is owned and operated by LFV [21]. In 2005 over 17 million passengers passed the airport and it had about
234 000 aircraft movements of which about 134 000 were international relations.
Approximately 16 000 employees work at the airport.
2.3.1 The airport’s runway system
An airport's runway system consists of runways and taxiways [22]. The strip where aircraft take off and land on is called a runway and the connecting pathways between the runways and the terminals are called taxiways. Movements of aircraft on these pathways are called taxiing.
2.3.2 Physical description of Arlanda airport
Due to the directions of the runways, Arlanda uses only two runways at a time during normal operations [22]. One for landings and take-offs, respectively, but in some cases only one runway is in use due to very bad weather conditions.
The Arlanda runway system consists of three runways.
Runway 1: 10 and 190 degree compass direction respectively, north-south direction. Length 3.3 km, width 45 m
Runway 2: 80 and 260 degree compass direction, respectively, east-west direction. Length 2.5 km, width 45 m
Runway 3: 10 and 190 degree compass direction, respectively, north-south direction. Length 2.5 km, width 63 m, the position of the runways is shown in Figure 2.
Figure 2 Positions of the runways at Stockholm Arlanda airport [22]
The distance between the parallel runways Runway 1 and Runway 3 is 2.3 km. The parallel runways enable parallel runway operations. There are three different kinds of parallel runway operations, independent, dependent and segregated. The most common operation in Arlanda is the segregated where the runway dedications are as described above during normal operations. Dependent and independent parallel runway operations are described further in section 2.7. The decision of which runway pairs to use depends on many factors such as wind direction, visibility and environmental conditions in the airport. Landings and departures should preferably be performed against the wind due to the lift force. The use of runway pattern might change several times during a 24-hour period, but can also remain unchanged.
2.4 Air Traffic Control Centre (ATCC) Stockholm
The Swedish airspace is divided into two areas controlled by Malmö ATCC and Stockholm
ATCC, including the area control in Sundsvall [23]. While Malmö ATCC has its main task on
en-route (high altitudes) traffic, Stockholm ATCC is currently focusing on traffic to and from Stockholm TMA and Arlanda airport. Stockholm ATCC uses the EUROCAT 2000 system and the subsystem Maestro to manage the traffic.
2.4.1 EUROCAT 2000
EUROCAT 2000 is an ATC system for processing radar and flight plan data [24]. A typical air
control workplace in this system has two monitors in different sizes [25]. All flights within the specific ATCC’s area are presented on the larger monitor, whilst the smaller monitor can present a list of all expected incoming flights within the zone of responsibility of the controller. In EUROCAT 2000 there are advanced functions that make sure that the flights are separated correctly. The controller can see if there is risk for a conflict between two aircraft 10
and on that occasion solve it by taking appropriate measures. An even flow of landing aircraft is a prerequisite to maintain a high capacity on airports such as Arlanda. This is accomplished by the controller through a planning function called Maestro. For a more comprehensive description, see section 2.4.2.
EUROCAT is constructed with a high amount of redundancy so that a number of errors can
occur without affecting the work of the controller. If, contrary to expectation, the whole system would break down, the controller has a picture of current flights on the other monitor generated by a separate system called Radar Ultimate Fallback Facility (RUFF).
2.4.2 Maestro
Maestro is a EUROCAT 2000 subsystem [1]. It is a multi-runway decision making tool for
airport TMA management [26]. Maestro aims to optimise the airspace and runway capacity by apportion the workload associated with Area Control Centre (ACC), approach and ground controllers, thus minimizing delays and extra fuel consumption. In order to achieve the objective, the system provides the ACC, approach and ground controllers with graphical interface of the calculated sequence and the control measures that have to be taken to appropriately manage the traffic. During this process, the controllers remain fully in charge of the sequence as the system allows them to make manual adjustments to test various sequencing options.
The Arrival Management (AMAN) of Maestro has several functions. One of them is to allocate each incoming aircraft to a destination runway considering runway allocation rules, restrictions considering noise and runway separation and flight priorities. Another is to optimize the overall sequence to minimize the global delay. AMAN also generates automatic coordination messages to relevant positions in case when the sequence is modified by one of the controllers. Maestro has also a Departure Management function (DMAN); however it is not covered here.
2.5 The system’s users – the aircraft
This section does not cover any technological aspects (e.g. engine specifications, steering, etc.) of the aircraft. Instead this section brings instead up selected topics of interest for the thesis.
2.5.1 Separation minima
Different types of aircraft, depending on their weights, generate different amounts of air turbulences, also known as wake vortices that can cause a following aircraft to lose its aerodynamic stability [27]. For instance, landing a heavy aircraft such as a Boeing 747 needs a relatively large distance to its trailing aircraft. On the other hand, an aircraft of the lighter type such as Cessna 404 generates relatively little air turbulence compared to heavy aircraft and therefore can the trailing aircraft have a smaller distance to the preceding aircraft. These rules for separation minima for all types of aircraft are presented in Table 1 with distances given in Nautical miles (1 NM = 1.852 km).
Table 1 Distance based separation minima rules by ICAO [9]
Heading aircraft Trailing aircraft
Heavy Medium Light
Heavy 4 5 6
Medium 3 34 5
Light 3 3 3
The different aircraft types are measured by the Maximum Take-Off Weights (MTOW). These are presented in Table 2.
Table 2 MTOW for different aircraft types Type
Heavy MTOW ≥ 136 000 kg
Medium 7000 kg < MTOW < 136 000 kg
Light 7000 kg ≤MTOW
2.5.2 Flight Management System
A Flight Management System (FMS) is a control and monitoring system used mostly in modern aircraft [34]. The system steers and monitors motors and navigation. The pilots feed flight regulating data into the FMS computer before take-off. Examples of such data are: starting point, flight route, flight levels, winds and temperatures on topical altitudes, take-off weight and fuel load. Using the data, the FMS sets the engines characteristics to the most economic setting. The setting changes automatically with reduction of weight of the aircraft and variations in flight level and air temperature. For navigational purposes FMS usually uses a combination of information from, among other things, ground radio beacons and GPS. On monitors the pilots can see a map with trajectory, break points with arrival times, radio beacons, airports with approach trajectories and holding. In addition, the pilots receive updated information from the system about the position of the aircraft. Figure 3 shows a part of an FMS.
Figure 3 An example of Flight Management System [5]
4 In case of Visual Meteorological Conditions – See Section 1.6 Definition list, the separation minima may be
reduced to 2.5 NM for medium-medium at least for aircraft heading to Arlanda airport.
FMS render possible to fly “Area Navigation” (RNAV5) and provides approach with better
precision [5]. The aircraft can currently with some FMS keep its track to the accuracy of three wing-widths and time of arrival to within six seconds anywhere in the flight plan [10]. The precision makes it possible to apply flight procedures that reduce the covered distance and reduce noise. In addition, the precision has the benefit of reducing fuel consumption. The
FMS integrates Required Navigation Performance (RNP6) with satellite-based navigation. 2.5.3 Precision Area Navigation
Basic RNAV7 has been mandatory in European airspace since 1998 [35]. Precision Area Navigation (RNAV) is the natural development from basic RNAV. A requirement for
P-RNAV is that an aircraft can keep its track with an accuracy of ± 1 NM (RNP 1) for at least 95
% of the flight time using advanced navigation databases, but normally the aircraft fly more accurately almost 100 % of the time. The P-RNAV equipment of the aircraft automatically determines the desired flight path by a series of waypoints in the database. Numerous aircraft today operating in passenger traffic can attain P-RNAV capability without having to implement additional onboard equipment. Initially RNAV will be executed in TMA.
P-RNAV will increase safety by introducing predictable and repeatable flight paths which enable
pilots and controllers to share the same knowledge of planned flight paths.
2.5.4 Standard Terminal Arrival Route
At many airports there often are approach routes defined by the local aviation authority as the route from an entry point partly or all the way down to final approach to a runway. A Standard Terminal Arrival Route (STAR) is a published flight route for arriving traffic, designated for Instrument8 Flight Rule9 (IFR) navigation and normally means direct approach to approved landing runway [2]. The correspondence for a departure is called Standard Instrument Departure (SID). An example of a STAR is shown in Figure 4.
5 Area Navigation - See Section 1.6 Definition list
6 Required Navigation Performance - See Section 1.6 Definition list 7 Basic RNAV - See Section 1.6 Definition list
8 Instrument – the instrument panel of an aircraft. 9 Instrument Flight Rule – See Section 1.6 Definition list
Figure 4 An example of a STAR [36] 2.5.5 The landing procedure
The most common landing procedure is called Instrument Approach Procedure IAP and covers from en-route phase to the landing phase [37]. An IAP normally starts at an en-route fix and ends at the IAF and is usually also marked in the STAR. The IAF is often the first navigational facility associated with the actual approach. However, the IAP is divided into four segments and can also be seen in Figure 5.
Figure 5 Projected view of the IAP [37]
1. Initial approach segment – this segment commences at the IAF and may be among other things a holding pattern or radar vectoring. The aircraft is manoeuvred to enter the intermediate section.
2. Intermediate approach segment – the aircraft should in this segment be prepared for the final approach by completing speed adjustment, positioning and doing pre-landing checks. The intermediate segment begins at Intermediate Fix (IF) and ends at the Final Approach Fix (FAF) and may be for example a procedure turn or a reversal turn in a holding pattern. 3. Final approach segment – The final approach segment for a non-precision approach
begins at the FAF and ends at the Missed Approach Point (MAP). The final approach may be made for a straight-in landing or a circling approach depending on the alignment of the final approach and the runway. A precision approach starts at the Final Approach Point (FAP), when FAF is not designated, which is a point where the segment 3 intersects the glide path for the precision portion of the approach. Descent to below minimum permitted altitude should not occur unless visual with the runway environment, such as approach lights and runway lights has been established.
4. Missed approach segment – If visual is not available upon reaching the particular point or minimum altitude a missed approach must be made. The MAP for a non-precision approach is defined by a fix, facility or timing. During the non-precision approach, the pilot may not descend below the Minimum Descent Altitude (MDA) unless visual. Once reaching the MDA, the pilot may continue to track inbound until reaching the MAP.
There are numerous types of instrument approaches where each approach has a separate and individual design criteria, equipment requirements, and system capabilities. There are other types of approach procedures such as visual approach. ATC can expedite aircraft managing by authorizing the pilots a visual approach instead of published IAP. To issue clearance, the controller must ensure that the pilots have the preceding aircraft or the runway in sight and verify that the pilots are responsible for separation and wake turbulence avoidance.
A typical approach from a horizontal point of view may look like Figure 6. Very often the aircraft descend to a given flight level from the controller and have to level-off in order to maintain altitude during the approach [38]. This procedure is repeated several times (6-10 times) before the final approach to the runway. Throughout the level-off phase the aircraft must add thrust in order to maintain the flight level.
Figure 6 A common arrival procedure of today [38]
When queues are forming the ATC leads the landing traffic into so-called S-curves with low speeds and altitudes, see Figure 6 [39]. The added thrust during the level-offs by the aircraft and traffic flying in S-curves gives rise to environmental impacts on both local and global level. The local impacts are due to the noise that aircraft carry along to communities in the vicinity of the airport. Global impacts are caused by non-optimal flight paths which lead to superfluous emissions of carbon dioxide that in its turn affects global warming.
Figure 7 Example of an S-curve seen from above
2.5.6 Aircraft Communication and Reporting System – ACARS
ACARS is a digital data link system transmitted via VHF which allows airline departments to
communicate with the aircraft in their fleet [40]. This system can be likened to “e-mail” for airplanes since the registration of each aircraft is its unique address in the system. ACARS provides routine items such as departure reports, arrival reports, passenger loads and engine performance data and other types of information to airlines and ATC from the aircraft. The system can be linked to the FMS which enables flight plans and weather information to be provided from the ground to air.
2.6
2.7
Capacity
Capacity can be interpreted differently depending on the situation. Janić (2000) defines capacity as it “refers to the throughput of a facility. It is the rate, at which users can be
accommodated.”
The capacity of an airport consists of a landside and an airside area if assumed that these are the sub-systems of the airport system [41]. These can be expressed by the maximum number of entities that can be accommodated in a given period under given conditions. The entities can be the landing and taking-off aircraft, arriving and departing passengers and their baggage and air cargo shipments. The focus only revolves around landing aircraft in the airside area as the relevance of the remaining subsystem and entity types is low for this thesis.
To describe the capacity of the airport airside area, the definition can be divided into several components [42]:
• The airspace around the airport enabling aircraft approaches and departures.
• The runway system where the aircraft spend its time before take-off and after landing. • A network of taxiways providing the departing and arriving aircraft access to the
runways before take-off and to the apron complex after landing, respectively.
The runway system with its adjacent airspace has a critical influence on the capacity of an airport (airside area) since it has the role of an entrance and exit to and from the gates respectively. As described above, the capacity can be expressed as the maximum amount of aircraft operations that can be carried out during a given period, i.e. an hour under the conditions of constant demand for service. By observing aircraft flows through the airport system it appears that the specific components, stated in the list above, of the airside area are functionally connected in a serial order. It is necessary to balance the capacities of each of the components to prevent the occurrence of periodical or permanent bottlenecks in the system, which may cause significant congestions and delays.
Investigations of airport capacity indicate that the capacity on the airside area is dependent on several factors. Some of the factors are constant while others change frequently. The most important constant factor is the design of the airport characterized by the number of directions of runways, taxi-ways and runway instrumentations. There are several kinds of variable factors where the meteorological conditions such as visibility and wind direction belong to the most unstable factor influencing the airport airside capacity.
Parallel runway operations
As described in section 2.6, the capacity at busy airports will not be sufficient to offer their services for the expected increasing air traffic. A known approach to partly solve this problem is by initiating parallel runway operations. Parallel runway operations can be divided into 3 modes with sub-modes defined by ICAO [43].
1. Simultaneous parallel approaches
o Mode 1 – independent parallel approaches: simultaneous approaches to
parallel instrument runway10 where radar separation minima are not prescribed between aircraft using adjacent Instrument Landing Systems (ILS) or
Microwave Landing Systems (MLS)11. The minimum distance between the
runway centre lines of the runways is recommended to be 1035 m [44]. 10 Instrument runway – See Section 1.6 Definition list
11 ILS and MLS – See Section 1.6 Definition list
o Mode 2 – dependent parallel approaches: simultaneous approaches to parallel instrument runways where radar separation minima between aircraft using adjacent ILS or MLS are prescribed [43]. The minimum distance between the runway centre lines is recommended to be 915 m [44].
2. Simultaneous parallel departures
o Mode 3 – independent parallel departures: simultaneous departure for aircraft departing in the same direction from parallel runways [43]. The minimum distance between the runway centre lines for Mode 3 and Mode 4 is recommended to be 760 m [44]. When the spacing between two parallel runways is lower than the specified value dictated wake turbulence considerations, the runways are considered as a single runway with regard to separation between departing aircraft
3. Segregated parallel approaches/departures
o Mode 4 – segregated parallel operations: simultaneous operations on parallel runways where one runway is used for approaches and one is used for departures.
A mixture of the modes might occur in some airports. For instance where one of the runways is used exclusively for approaches while the other runway is used for both approaches and departures. This is called semi-mixed parallel operations. As the distance between the parallel runways at Arlanda airport is larger than the recommendation by ICAO for Mode 1, can any presented mode of parallel runway operation be performed at the airport. Arlanda is currently performing Mode 4 when the weather conditions are appropriate. Mixed mode means that the parallel runways are used independently of each other which means that two independent landings (take-offs) can occur on the runways at the same time. By using the terminology above, the mixed mode can be described as a mixture of mode 1 and 3.
2.8 Shortages of the present system
The airports are today the bottlenecks in the air traffic system [45]. By the year 2020, the amount travelling by air within EU is expected to double. To meet the expected demand one natural solution is to build more runways/airports. However, due to public concerns, e.g. noise and environmental issues, the planned expansion of runways/airports will not be sufficient to face the expecting demand. Furthermore, a simulation study with increased traffic volumes using existing systems and procedures has shown some alarming results, such as unacceptable aircraft holding times and non-optimal flight paths for aircraft. The actual procedures are bounded up with many problems. A review of the problems is listed below.
• Bad precision and predictability
Since the flight path is not known in advance, it is difficult for all parts to predict the ETA. This leads to confusion in ground activities, low quality of service and high costs for contributing resources.
• High fuel costs
The aircraft cannot calculate optimal descent profiles by reason of unknown flight paths. This gives rise to non-optimal descents and extra fuel usage. Today, the fuel costs are approaching 15 % of total cost of SAS [46].
• Environmental impacts
Queuing traffic on low altitudes causes noise and different kinds of emissions [45].
• Limited communication between participants
The present air traffic system is built up so that each part of the system is optimised on the basis of its own activity [39]. The strategic information which the participants of the system possess is limited. For instance when pilot and ATC have agreed and confirmed a new ETA, the information is not communicated to the ground personnel. Due to the lack of optimal information exchange the airlines and airports are in many cases forced to use extra personnel to cover the time uncertainty.
3 Research
This chapter covers research related to the concept of time based sequencing.
3.1
3.2
The NUP II+ project
The NEAN Update Programme Phase II+ (NUP II+) project is aiming to develop applications with the potential to provide benefits to the ATM domain [47]. NUP II+ is based on Trans-European Transport Networks (TEN-T) research projects known as North Trans-European ADS-B12 Network (NEAN) [48]. Predecessors of NUP II+ are NUP Phase I and II whose objectives were to search and gain understanding of the potential benefits with supporting technologies and operational processes of ADS-B. The focus of the NUP II+-project is, unlike previous phases, to validate a set of applications based on ADS-B and 4DTR data in live trials with the purpose to achieve an operational introduction. SAS has been participating in the project since January 2005.
The objective of the project is to evaluate if the selected applications have the potential to present operational and safety benefits to the participants of the project and to provide a path towards the direction of large scale validation of the selected applications in European airspace [47]. The project has, apart from SAS, the following participants: LFV Group, Austrocontrol, DFS, Eurocontrol Experimental Centre, Lauda Air, Rockwell Collins/France, Smiths Aerospace, AVTECH, Com4Solutions, Boeing Research & Development and Airbus.
NUP II+ is to 50 % financed by the European Commission and the remaining 50 % is
financed by the participating partners.
The Green Approach trials
The 19th of January, 2006 was the first time in history a Green Approach was executed during
a commercial flight. A Boeing 737 departed from Luleå and landed at Arlanda with a deviation of two seconds [49]. There are two reasons why this has not been done before [50]. The first reason is that the newer aircraft can generate and assure very accurate data using satellite navigation, the other is the ability to communicate in a way that has not been done before. The data from the aircraft can be transmitted to the ATC via data links and the most important factor is the drive to increase capacity and a more environmental friendly operation from a political point of view.
A Green Approach is basically an uninterrupted idle approach from top of descent (ToD) to the runway threshold with the purpose to minimise fuel burn and environmental impact [51]. When the aircraft reaches ToD it follows a, by the FMS calculated, flight path to the runway. Figure 8 shows a schematic overview of a Green Approach, compare with traditional approaches in Figure 6. Interventions from the controller are basically made only when the flight safety is jeopardised.
12 ADS-B – See Section 1.6 Definition list
Figure 8 Schematic overview of a Green Approach [38]
A Green Approach is optimised from the perspective of the aircraft’s performance. The application builds on initial 4DTR (see section 3.2.1) downlink with ETA for threshold of the dedicated runway. The approach path is defined as a P-RNAV STAR13
during the entire way to
runway threshold. Of the whole SAS aircraft fleet only Boeing 737 New Generation are capable of transmitting a 4DTR to fly a Required Time of Arrival14 (RTA) since they are equipped with a newer version of FMS.
Apart from environmental benefits, the Green Approach will allow airlines to improve planning of the aircraft turn-around15 process on the ground because of the accuracy of the down linked ETA. In addition, the cockpit crew will be able to have a better focus on the planning and execution of the approach as the planned approach profile will be known far in advance and as the controller is only expected to intervene in order to maintain the safety. Benefits for the airport, due to the Green Approach, may include better planning and utilisation of ground personnel.
Since March 2006, 800 Green Approaches have been performed [52]. For the year 2007 the ambitions are to perform 2000 Green Approaches to Arlanda. During the first year of trials, approximately 100 kg of jet fuel was saved per each flight. Essentially, the fuel savings are the main reason why EU is supporting the project. When the majority of the approaches to Arlanda in the future are Green Approaches for SAS, which is a middle-size airline flying to a middle-size European aerodrome, the fuel savings would be up to 40 million SEK. Recall that SAS fuel costs are approaching 15 % of the total costs. Recall also that the number of aircraft movements in 2006 were around 234 000, a number that is likely to be increased. If each of those landings (half of the movements) requires at least 100 kg less fuel, a considerable environmental effect is achieved by reducing emissions of carbon dioxide and other substances of harm for the environment.
When discussing Green Approach a parallel can be drawn to the idea about ECO-driving for road traffic. The way of driving affects the fuel consumption [72]. A considerable amount of
13 P-RNAV STAR is described as an uninterrupted path to a waypoint e.g. IF, FAF etc [74]. 14 Required Time of Arrival – See Section 1.6 Definition list
15 Turn-around – The time between landing and departure when aircraft are on the ground and often includes e.g.
loading/unloading, refuelling and cleaning.
gasoline and money can be saved by driving in a certain way. Among other things ECO-driving includes careful planning of the route and rolling towards e.g. intersections and traffic lights with idle thrust and without declutching. The idea is the same for Green Approaches. The flight path is known in advance and the aircraft performs an approach with least possible thrust. A Green Approach can, using the same terminology, be characterized as ECO-flying.
3.2.1 Four-dimensional Trajectories – 4DTR
4D trajectories are essentially a list of data over aircraft movements over certain
pre-programmed coordinates based on the current location of the aircraft [53]. They are generated when a request from CIES (see section 3.3.2) is received by the FMS. 4DTR data contains information about an aircraft’s whole flight route described by the time, but also with altitude, latitude, longitude, point type and turn radius and turn direction [54]. An excerpt from a 4DTR data may appear as following: “0,L934,N59302E017443,1279,090456”. The order of the data is; point type, turn direction, turn radius, lat/lon, altitude, time. For the trajectories sent to
CIES, they can be illustrated for the user in a 2D-image shown in Figure 9.
Figure 9 4DTR displayed on CIES [39]
3.3 Definition of time based sequencing for approaching aircraft
Time based sequencing is an innovation in the ATC domain. In a time base sequenced system aircraft are categorized depending on their navigational performance. One category is aircraft capable of performing Green Approaches, described in previous section. Currently, only a few
of these high precision aircraft are operating in commercial traffic. The amount of aircraft capable of performing Green Approaches is expected to increase significantly.
The other category is all other aircraft with less advanced navigational performance, designated as non-precision aircraft. Both types of aircraft are sequenced by time to different points in the flight path. The Green Approach aircraft are sequenced directly to the runway, whereas the non-precision aircraft are sequenced to the entry points.
3.3.1 Description of the concept
The approaching aircraft fly through appointed entry points and via P-RNAV STARs to the runway [39]. Speed limits in the approach paths are necessary. Recall that P-RNAV implies that aircraft keep its track with an accuracy of ± 1 NM horizontally for at least 95 % of the flight time. ETA is calculated by the ATC system when flights are 30-40 min away from the
airport. When queues are forming up in the system, several flights have ETAs that are close to each other. ATC’s sequencing aid, CIES, allocates feasible ETAs in order to establish a stable inbound flow approximately 30 min before Actual Time of Arrival (ATA) [53]. If the originally calculated ETA becomes postponed, a Revised ETA (RETA) is communicated to the participants of the airport. In case of fixed queues there is no reason to change the mutual landing order. The RETA applies for the participants of the airport to plan their resources. There are two ways for ATC to communicate with the aircraft, either via radio telephony and/or data link depending on the type of aircraft [39]. Aircraft equipped with data link and the newer version of the FMS have the capability to send their remaining flight profile (4DTR) and receive an RTA with a landing precision of ± 10 s. The system will then optimize the descent profile of the flight by taking the new time constraint into account. Other aircraft are allotted an EAT (not ETA) via radio telephony calculated by CIES. The concept is illustrated in Figure 10.
Figure 10 The concept of time based sequencing
For adaptation of the EAT the pilot adjusts the flight velocity and/or are radar vectored to pass over the entry point with an estimated time precision of ±30 s and an as far as possible optimal descent profile. During the en-route phase of the flight and from ToD to landing, the aircraft can approximately delay the flight with, respectively, five minutes and one minute. The controllers adjust the speed or designate alternative courses (by radar vectoring) to the pilots so that separation minima between the aircraft are achieved.
The adaptation of EAT for non-precision aircraft can be seen from another perspective. From the point when the pilots receive the EAT to the entry point there is an approximate distance in time, that the aircraft is intended to keep, see Figure 11. This time can be likened to lead time in the logistics domain. Lead time is the time span from order reception to delivery of the order to the customer [55].
When an aircraft is heading to an airport, suppose that the present time is 11.40 and the pilots have received an EAT to 12.00. The time distance is therefore 20 min (lead time). However, assume that the aircraft has a velocity that occasionally is too high. This leads to a passage above the entry point that is too early. In case of a too high velocity the aircraft adapts to the
EAT according to the description above. The precision of the adaptation is, using the lead time
simile, equal to the interval [19:30, 20:30] min. A similar parallel can be drawn for Green Approach aircraft.
